There are numerous applications in which electrical contactors are used to isolate an electrical circuit from an electrical power source. In many of these applications, the contactor is operated in what is sometimes referred to as a "dry-switching" mode, i.e. in a mode in which no current is drawn through the contactor tips at the time of opening or closing. For example, it is common to use a key switch controlled contactor in electrical vehicles to isolate the battery from the electric traction motor and the associated control when the key switch is turned to an off position.
While there are advantages to operating a contactor in a dry switching mode, such as, for example, to extend contactor tip life by eliminating arcing, there is also a significant disadvantage. In particular, contactor tips are typically produced from a silver based metal and will form a surface oxide or sulfide, which have poor electrical conductivity and act as an electrical insulator at the tips. In some instances, the oxide or sulfide build-up may accumulate sufficiently to block current flow to the electrical system while a lesser accumulation may reduce available power to the system.
One method of avoiding oxide and sulfide build-up is to construct contactors in which the tips exhibit a wiping action during operation, i.e., a moving tip that rubs across a surface of a stationary tip to wipe the oxide from the tip. A disadvantage of such contactors is their relatively high cost in comparison to conventional contactors.
In U.S. Pat. No. 5,502,609, an electrical circuit for preventing contactor tip contamination is shown. The circuit shown utilizes an electromagnetic coil to actuate the contactor by drawing current from a capacitor bank which is in parallel connection to the voltage source. The voltage difference between the contactor tips when closing is related to the amount of current which has been discharged by the capacitor. This circuit relies directly on the capacitor discharge to create a voltage difference between the contactor tips. Due to the reliance on capacitors, the voltage difference between the contactor tips will vary with parameters, such as the temperature of the circuit components, the discharge rate, the capacitance, and the resistance of the actuating coil through which the capacitor discharges.
Accordingly, it is desirable to provide a means for preventing oxide and sulfide build-up on contactor tips which does not require special contactors, and which provides a relatively constant and predictable voltage difference across the contactor tips.